1. Field of Invention
The present invention relates to apparatus for displaying images, and particularly relates to apparatus for displaying images with compression mechanism.
2. Description of Related Art
With top image quality in display, the LCD, Liquid Crystal Display has become prevailing popular in mass applications including monitor, screen of note book computer, display screen of mobile phone, PDA, GPS, e-dictionary, . . . etc. Digital TV and HDTV are adopting LCD display as its display screen. The digital image and motion video have been adopted in an increasing number of applications, which include digital camera, scanner/printer/fax machine, video telephony, videoconferencing, surveillance system, VCD (Video CD), DVD, digital TV . . . etc. The success of development of the digital image and video compression standards fuels wider applications in digital display devices.
FIG. 1(a) is a diagram of a display system. In the display system, an image source device 10, e.g., a MPEG/JPEG decoder, supplies one or a series of raw image(s) to a display control chip 12. The display control chip 12 adapts the raw image(s) and supplied adapted image(s) at right timing to a display driver device 14 that renders an output image on a display device 18, e.g., a LCD panel. For adapting the raw image(s) to the display device 18, the display control chip 12 has a control logic unit 120 and several manipulating devices. In this example, the display control chip 12 has a de-interlacer 124 and a scaler 126.
The de-interlacer 124 de-interlaces interlaced images into a restored image. For example, in MEPG2 standard, odd lines and even lines of an image are interlaced and separately compressed and therefore, a de-interlacer 124 is necessary for restoring two portions of an image into one complete image. To improve the image quality when concatenating two separately compressed portions, the de-interlacer 124 needs to reference to at least 4 corresponding images, e.g. previous frames. In fact, some de-interlacer 124 needs 6-8 reference images to achieve better image quality. A cache 122 is embedded in the display control chip 12 for temporarily storing operation data. However, one or more off-chip memory 16 is usually necessary for storing reference images. A memory interface 128 of the display control chip 12 is therefore disposed for accessing the off-chip memory 16.
Because the de-interlacer 124 needs to access reference frames so frequently, the channel between the display control chip 12 and the off-chip memory 16 needs high bandwidth, which consumes higher power and complicates the design of the display system. In addition, the de-interlacer 124 and the scaler 126 usually needs to reference 4-10 lines during operation. The on-chip cache 122 usually dominates 30% area of the display control chip 12, which is around 750K logic gates under 0.25 um CMOS process. Therefore, the size of the on-chip cache 122 is also a cost factor of designing the display control chip 12.
FIG. 1(b) illustrates a diagram illustrating the relationship of the display driver 14 and the display device 18. The display driver 14 has a plurality of column drivers 17 and row drivers 19 for providing driving currents to the display device 18 for rendering output images thereon. In general application like LCD displays, the display driver device 14 is designed as a couple of driver chips that read a memory buffer storing the output image to be displayed. Though the display control chip 12 has dealt with the timing, the display driver 14 needs a frame buffer that stores the output image. The frame buffer is also a cost factor during designing the display system.
For mobile phones or other portable devices, a single chip deals with functions provided by both the display control chip 12 and the display driver device 14. In such application, the frame buffer in the display driver device 14 mentioned above is particularly a critical issue to be addressed.
Therefore, it is very beneficial to improve the display system to downsize both the on-chip cache and the off-chip memory while keeping same image quality.